FAA Powerplant Written Practice Test

What is the consequence of having a richer mixture than required at cruise settings in an aircraft engine?

1. Improved efficiency

2. Potential fouling of spark plugs

3. Enhanced throttle response

4. Higher fuel economy

The correct answer is: Potential fouling of spark plugs

A richer mixture than required at cruise settings in an aircraft engine typically leads to the potential fouling of spark plugs. When the fuel-to-air ratio is excessive in favor of fuel, it can result in incomplete combustion within the combustion chamber. This excess fuel can lead to carbon deposits accumulating on the spark plugs. Over time, these deposits can interfere with the proper operation of the spark plugs, which may cause misfires or rough engine performance. The other choices do not hold true in this context. For example, improved efficiency and higher fuel economy are generally associated with an optimal fuel-air mixture, not a richer one. An enhanced throttle response might also not be a benefit of a richer mixture; it can lead to sluggishness or poor performance instead. Therefore, the significant downside of having a richer mixture is the potential for fouling spark plugs, affecting engine reliability and performance.